The present invention primarily relates to a glass door merchandiser type of refrigerated display case. While reference is made herein to the use of glass doors since these are the types of doors most frequently utilized, other types of doors can be used within the scope of the present invention. In addition, the term refrigerated, in accordance with the present invention, is intended to incorporate both those cases maintained at a temperature either at or in excess of 32.degree. F., such as display cases utilized for displaying milk and fresh foods, and those cases maintained below 32.degree. F., such as frozen food cases.
During the operation of glass door refrigerated merchandiser display cases, there is a tendency for condensation to accumulate between the two glass members mounted in the door frame. In order to minimize such condensation, electric resistance heaters are mounted outside the door. The electrical heaters apply sufficient heat to the glass members and to minimize the condensation occurring on the glass. The use of such electrical resistance heaters give rise to several disadvantages. First, the heaters utilize a substantial amount of electricity thereby increasing the cost of operation. In addition, the application of heat to the glass door also will cause heat to be conveyed into the display case by conduction thereby increasing the refrigeration load and effectively decreasing the efficiency of the case.
Present glass door merchandisers use only one air conduit, this being the refrigeration air conduit in which the evaporator coil is located. For low temperature applications, the air curtain across the access opening of the merchandiser can run as low as -20.degree. F. With heavy customer usage, which results in frequent openings of the door, the heat and mass transfers that penetrate the air curtain become substantial thereby increasing the refrigeration load.
In order to prevent condensation from forming on the outside of a refrigerator or freezer door, U.S. Pat. No. 4,009,586 discloses passing a stream of warm air. The warm air produced by the compressor of the refrigerator is expelled from a location below the door in an upward direction along the outside of the door.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation, it is possible to avoid any significant frost buildup within the display case.
Typically, within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The three approaches include: utilizing electric resistance heater; passing a compressed gaseous refrigerant through the refrigeration coils; and, circulating ambient air through the air conduit. Due to the increasing cost of energy in recent years, efforts have been made to place more emphasis on the utilization of ambient air defrost systems in place of the electrical resistance heaters or compressed gaseous refrigerant defrost systems.
One type of system that employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,403,525, 3,850,003 and 3,937,033, all to Beckwith et al. These systems use fans separate and distinct from the main circulating fans. The additional fans are turned on only during the defrost cycle of operation for pulling ambient air from outside of the display case directly into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display case. Such ports are normally closed during the refrigeration cycle and are opened during the defrosting cycle. The Beckwith, et al. patent U.S. Pat. No. 3,850,003 indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
Another type of ambient air defrosting system is shown in U.S. Pat. No. 4,144,720 issued to Subera, et al. which is assigned to the same assignee as the present application. In the foregoing patent application, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this system, the direction of air flow within one of the conduits is reversed, for example, by the use of reversible fans for ambient air defrost. U.S. Pat. No. 4,026,121 to Aokage, which illustrates an open front display case, and U.S. Pat. No. 4,120,174 to Johnston, which illustrates an ooen top display case, also disclose reverse ambient air flows for defrosting.
In those ambient air defrost systems disclosed in the above-noted patents which use a reverse air flow, during the defrost cycle of operation, ambient air can easily be drawn through the access opening into the air conduit through the outlet opening of the air conduit and then expelled from the air conduit after the defrost operation through the inlet opening and out of the cabinet through the access opening. Such an arrangement, however, cannot be readily used in a glass door type merchandiser refrigerated display case, since the front opening in the cabinet is covered by the doors. Thus, in order to employ an ambient air defrost system, a different type of system had to be developed.
In seeking to employ ambient air defrost techniques in a glass door case, systems have been developed for drawing in air over a limited portion of the air conduit by opening flaps to the conduit, which flaps are arranged so as to astraddle the evaporator coils of the refrigeration mechanism; such systems are disclosed in U.S. Pat. Nos. 3,226,945 to Spencer and 4,072,438 to Johnston. The patent to Spencer illustrates a plurality of different embodiments of open top refrigerated display cases, both of a single shelf and multishelf type, in which a glass cover is arranged over the opening in the display case. During the refrigeration cycle of operation, air is drawn through the evaporator coils by a positive pressure created upstream of the coils; such air after being refrigerated is circulated through the air conduit and into the display section of the case. The patent to Johnston discloses a glass door type merchandiser display case in which air is circulated through the air conduit and through the evaporator coils arranged within the air conduit in such a direction that cold air enters from the bottom of the opening in the cabinet and after passing across such opening is then drawn back into the air conduit by an air inlet located at the top of the opening in the cabinet. Such systems are relatively complex and can involve certain operational problems.